Bird vocalizations are produced by the syrinx. In songbirds this structure consists of two sound sources, one lodged in each bronchus. Each of these sound sources has its own complement of muscles innervated by the tracheosyringealis branch of the ipsilateral hypoglossus. In canaries, chaffinches, and white-crowned sparrows most or all song components are under left hypoglossal control. On this basis the left hypoglossus is described as dominant for vocal behavior, without any exceptions encountered so far (N greater than 40). Occurrence of this phenomenon will now be sought in oscine songbirds with very simple song, such as the field sparrow and chipping sparrow, and in non oscine songbirds, such as the phoebe. In time we hope to understand the evolutionary significance of hypoglossal dominance. Our interest in hypoglossal dominance has also led us to inquire into the central pathways controlling hypoglossal function. Over the past two years we have developed an atlas of the canary brain and commenced to plot brain areas involved in vocal control. This plotting will continue, and the effect of unilateral and bilateral brain lesions on song will be fully described. We hope to establish to what extent, if at all, the two halves of the brain show functional asymmetries comparable to those described at the hypoglossal level. Thesis work by A.P. Arnold in the P.I.'s lab. has furnished evidence suggesting uptake of testosterone at various levels of the brain's vocal control network, including motor neurons. Recording and stimulation of small groups of neurons in this network should help clarify the role of testosterone uptake by those neurons. Recording and stimulation techniques will also be used to dissect the functional organization of brain vocal control areas. This approach too is expected to shed light on the central pathways underlying the observed hypoglossal dominance.